The invention relates to a process for the installation and tensioning of a brace having a false bearing, for example a stay cable for a cable-stayed bridge, an external tensioning member or similar comprising a bundle of plastic-sheathed individual elements made of steel such as, for example, wires, strands of wires or similar, and anchoring devices suitable for carrying out this process.
In many cases braces such as those used in the construction industry for anchoring building components, for example as stay cables for cable-stayed bridges, external tensioning members or similar bracing elements, consist of a bundle of individual elements such as steel wires or strands of wires which are positioned together in a tubular sheath in the free area of the brace, passed through the relevant building components and anchored at the end opposite the point of entry therein by means of anchoring devices. These anchoring devices generally consist of an anchoring plate with holes through which the individual elements are passed. The holes have initially a cylindrical and subsequently a conical area in which the individual elements are anchored by means of multiple ring wedges. The free area of the tubular sheath around the brace may be made of a plastic such as polyethylene (PE) or of a steel tube. The anchoring area of the sheath generally consists of a steel anchoring tube.
Strands of steel wires provided with a corrosion-proof coating of grease and a plastic sheath, generally of PE, are often used as the individual elements for braces of this type. This sheath may be extruded and thus fit tightly around the strand in the form of a tubular sheath, a so-called PE jacket, and move with it when the strand is tensioned, or it may surround the strand at a certain distance in the form of a tube. In this case, the strand is pulled out of the jacket during tensioning. While PE-sheathed strands of this type can be used in the same way as naked, unsheathed strands it is always necessary to expose the strands by removing the PE jacket in the area of the anchoring devices so that the ring wedges used for anchoring can engage directly with the metal surface of the strands.
The ends of the strands intended for anchoring are often exposed by removing the PE jacket prior to installation in the building structure in question. Here it is often difficult to determine exactly prior to installation the length along which the PE jacket must be removed in order to anchor the strands properly. If the exposed length proves to be too short, the reliability of the anchoring may be jeopardized. If on the other hand the exposed area is too long, the reliability of the anti-corrosion protection in this area may be jeopardized.
In order to remove a tightly fitting PE jacket from the strand irrespective of the tolerances involved in determining the required length and of structural inaccuracies in such a manner that in its final state, i.e. once the strand is tensioned, the remaining PE jacket ends as close as possible to the anchoring wedges, a method is already known whereby the PE jacket on each individual strand is removed along the length of the extension which occurs at the tensioned end during the tensioning processes (DE 197 33 822 A1). To this end, a jacket-removing tool is positioned in the area of the anchoring of the strand from which the tensioning process is to be carried out and used to remove the PE jacket during the tensioning process as a result of the longitudinal movement which occurs at the tensioning end when the strand is tensioned. It is generally sufficient to slit the PE jacket longitudinally so that it can then be separated from the remaining part of the jacket by means of an annular cut. This process is largely successful in avoiding errors due to structural tolerances and eliminating the need for costly measurements. However, the jacket-removing tool required to carry out this process is costly.
Set against this background, the object of the invention is to find a simpler and less costly method of removing the PE jacket in the area of the tensioned end or of exposing the strand in this area in such a manner that, in its final state, the remaining PE jacket ends as close as possible to the area in which the strand is anchored.
As disclosed in the invention this object is achieved by means of the process described in claim 1.
Two independent anchoring devices suitable for carrying out this process for this type of strand are indicated in claims 7 and 8.
Advantageous developments are detailed in the sub-claims.
Rather than removing the plastic jacket which fits tightly around the individual elements of a brace at the end at which said individual elements are tensioned and anchored along the entire length of the extension which occurs during tensioning, the basic idea behind the invention is to prevent said jacket from making the longitudinal movement in the anchoring area caused by the tensioning of the individual elements and thereby compressing it in the area prior to anchoring in so far as it follows the longitudinal movement of the strand during tensioning. During this compression the plastic jacket undergoes first elastic and then, at least in part, plastic deformation.
There are several possible methods of preventing the plastic jacket from moving longitudinally. A first possible method consists of inserting into the cylindrical part of the holes in the anchoring plate a compression tube which tightly surrounds an individual element and one end of which forms a stop for the end of the plastic jacket, while the opposite end lies adjacent to the thinner end of the anchoring wedge.
Another possible method consists in forming a shoulder in the area of the cylindrical part of the holes which penetrate the anchoring plate in the form of a blind hole extension against which the end of the plastic mantel then abuts. Since the diameter of the part of the cylindrical hole adjacent to the wedge then has to be the same as the external diameter of the individual element, this does however present the disadvantage that during assembly the individual elements cannot be inserted through the anchoring plate into the tubular sheath. Rather the anchoring plate has to be placed in position from outside after the individual elements have been introduced.
In many cases, a spacer made of plastic having holes through which the individual elements can pass is positioned behind the anchoring plate on the structure side. This permits a third possible method in which the shoulder forming the stop can be provided on this spacer which is in turn supported by the anchoring plate.
The extent of the possible compression of the plastic jacket is dependent upon the properties of the materials involved, the temperature and other factors. For this reason it may be useful, in particular in the case of long tensioning distances, to expose in advance a certain area of the end of the strand at which it is to be tensioned and anchored, either by removing the plastic jacket prior to installation or by removing the jacket during the tensioning process and simply using the compression process disclosed in the invention for fine tuning in order to extend the end of the plastic coat and thus the anti-corrosion protection as close as possible to the anchoring device.